The present invention concerns a method for determining the reclose time of a circuit breaker on an electric network comprising a three-phase transmission line and a high voltage source. The method of the invention is more particularly adapted to extra high voltage lines, i.e. with a nominal source voltage of several hundred kV.
Such an electric network can be modelled according to a first approximation by the equivalent circuit shown FIG. 1. The network 1 comprises:                a voltage source S,        a three-phase transmission line L,        a circuit breaker 6,        a shunt compensation reactor 5,        a capacitive voltage transformer 4.        
The source voltage S is a very high voltage having a nominal value of 500 kV for example and a network frequency of 50 Hz.
The three-phase transmission line L is a line of length 400 km for example enabling the transport of three phases A, B and C.
By circuit breaker is meant both a circuit breaker commanded by three independent single-pole commands each associated with a phase, and a circuit breaker commanded by a single three-pole command. Generally the three-phase circuit breaker 6 comprises at least three cut-off chambers each associated with one of phases A, B or C. If nominal voltage is high, several cut-off chambers may be connected in series. The circuit breaker 6 therefore comprises at least three pairs of contacts, each pair being associated with one of the three phases of line L and making it possible to interrupt any current circulating between source S and line L by separating the two contacts in the event of a fault on the associated phase, the first contact being on the source side and the second contact being on the line side. Only two contacts 7 and 8 associated with a phase of the circuit breaker 6 are shown in FIG. 1.
The shunt compensation reactor 5 is an inductance coil for example allowing compensation of capacitive reactive power on long, high-voltage, electric power transmission lines.
The capacitive voltage transformer 4 located at the start of the line is used to measure voltage on the line side of the circuit breaker.
A sudden change in configuration of the energy transport network generated by the functioning of a circuit breaker causes a rapid transient overvoltage, called switching surge which propagates over the network. These switching surges may occur on the tripping or reclosing of circuit breakers. Since the use of circuit breakers without re-enabling has become generalised (i.e. with automatic re-setting of the circuit breaker after tripping) it is on closing and especially on reclosing a line still containing a trapped charge that the highest overvoltages occur.
A first solution to this problem consists of using an auxiliary system comprising a so-called closure resistance in series with a pair of auxiliary contacts, said auxiliary system being mounted in parallel with the cut-off chamber. The auxiliary contacts are actuated a few moments before the contacting of the main contacts so as to insert the closure resistance in the circuit. With this two-step triggering it is possible to reduce closure over-voltages with great efficacy.
This first solution, although very effective, has the drawback of being very costly.
A second solution consists of controlling the reclose times of the circuit breakers using electronic synchronization devices to replace the closure resistances. Said devices allow synchronized switching of a high voltage transmission line.
Therefore, when a single phase fault occurs (which account for more than 90% of line faults) on a high voltage line it is possible in certain networks that the elimination of the fault involves three-pole opening of the circuit breaker followed by almost immediate re-closing (between 300 ms and 1 s) in the endeavour to obtain ensured continuity of service. In this case, two of the phases are therefore vacuum switched by one of the end-of-line circuit breakers. On reclosing, the circuit breaker contacts must be closed at the right time (i.e. substantially at the time when the voltage at the contact terminals of the circuit breaker is zero) on these two vacuum phases, so as to limit overvoltages to an adequate value. This moment varies according to network configuration and must be determined by a closure algorithm in relation to the voltage signals measured on the network and supplied to the algorithm. The choice of right moment for reclosure is based on analysis of the voltage, at the contact terminals of the circuit breaker, of each healthy phase.
However, the implementing of this second solution also raises some difficulties.
For example the lines compensated by shunt reactance have the characteristic, after opening, of oscillating at a frequency in the order of 50 to 90% of the network frequency. This oscillation frequency is chiefly related to line capacity and its shunt compensation reactance. The voltage occurring on the terminals of the circuit breaker therefore shows greater or lesser beats depending upon the extent of compensation which varies in relation to transmitted power. During synchronization on reclosure, zero beat must be targeted to limit overvoltages. The determination of this zero beat is not easy insofar as, in practice, the faulty phase influences the signal of the two healthy phases so that the signal obtained which is to be analysed at the contact terminals of the circuit breaker of the two healthy phases is of very complex shape and difficult to analyse. Consequently results may not reach the desired accuracy of synchronization. Obtaining a satisfactory result i.e. at a determined confidence level, requires a relatively long convergence time for the closure algorithm used.